Recent development of the national economy has been accompanied with the increasing demand for electric power and the diversity of loads to the electric power. In this connection, there is an increasing need for flexible electrical power utilities that are effectively adaptable to variations of supply voltage. Until now, voltage regulator-transformers provided with tap-changers shown in FIG. 18 has been usually used for stabilizing voltage in the electric power supply systems. The tap-changing type voltage-regulating transformer, however, has intrinsic limitations on its maintenance and service life because it includes a tap-contactor and tap-switching mechanism that cannot avoid mechanical faliure and abrasion of its moving parts causing wearing of parts, poor contact, time-lag of voltage and so on.
In other words, any conventional voltage-regulator-transformer incluing a tap-changer involves a maintenance problem of mechanical wearing of tap-contacts and a tap-switching mechanism and a performance problem of time-delay in voltage control due to slow action of the switching mechanism.
In view of the foregoing, the present invention was made to provide a flux-controlled variable transformer that can control the voltage at a high speed without using a voltage-adjusting tap-changer.